This invention relates to the processing of monolithic integrated circuits. More particularly, the invention relates to forming isolation structures in a substrate of a monolithic integrated circuit.
Many monolithic integrated circuits contain millions of active devices, such as complementary metal-oxide semiconductor devices, formed in a semiconductor substrate. Typically, regions of nonconductive material, such as silicon oxide, physically separate these active devices in the substrate and provide electrical isolation between neighboring P- or N-doped regions of the active devices. These regions of nonconductive material are often referred to as isolation structures. One type of isolation structure is a shallow trench isolation structure. Methods of forming such structures are generally referred to as shallow trench isolation processing.
Past shallow trench isolation processing methods have generally included depositing an oxide material that covers the substrate, such as by Thick Trench Fill Oxide deposition. This oxide deposition step fills previously-formed isolation trenches in the substrate as well as depositing oxide on the substrate surfaces that lie between the trenches. To accommodate further processing of the active device structures, the excess oxide material is typically removed from the substrate surfaces that lie between the trenches. In prior shallow trench isolation processing methods, removing the excess oxide involves reverse masking and etching, followed by chemical mechanical polishing to planarize the remaining oxide material at the surface of the substrate.
A significant disadvantage of the prior methods is the necessity of the reverse mask and etching steps to remove the oxide between the trenches. Each photolithography processing step adds significant cost, processing time, and complexity to the fabrication process. For example, photolithography processing typically requires generation of a mask indicating the geometric pattern of the areas of excess oxide to be removed, application and curing of photoresist, exposure of the photoresist using the mask, and development of the photoresist after exposure. Not only are each of these steps time consuming and expensive to perform, but each step introduces a quality control concern. Thus, quality control inspection steps are typically included which add more cost and time to the process. Thus, masking and patterning steps are to be avoided whenever possible.
What is needed, therefore, is a process for forming isolation structures in a monolithic integrated circuit that eliminates the photolithography step required to reverse mask and etch the oxide between the isolation structures, and the costs attendant with such photolithography steps.
The above and other needs are met by an improved process for planarizing an isolation structure in a substrate. The improved process includes depositing a pad protective material over the upper surface of the substrate, and selectively removing portions of the pad protective material to expose portions of the substrate and to form sidewalls in the pad protective material. A trench is formed in the exposed portions of the substrate, and a trench fill material is deposited in the trench and over the pad protective material. A trench protective material is deposited over the trench fill material and in contact with the sidewalls of the pad protective material, such that the pad protective material and portions of the trench protective material together form a continuous protective material layer. Portions of the trench protective material and the trench fill material are selectively removed down to the level of the upper surface of the pad protective material. Finally, the pad protective material and any remaining trench protective material is removed, leaving the trench filled with trench fill material that is planarized at the upper surface of the substrate.
By forming a continuous protective material layer that completely covers the trench fill material in the trench, the trench fill material in the trench is protected during later process steps that nonselectively remove trench fill material lying outside the trench. In this manner, the trench fill material lying outside the trench may be removed without photolithographic masking and patterning steps. Thus, the process according to the invention reduces the cost and complexity of planarizing the trench fill material.
In one preferred embodiment of the invention, the continuous protective layer is a nitride layer that acts as a polish stop during a chemical mechanical polishing step for removing the unwanted oxide trench fill material.
In another preferred embodiment, the continuous protective layer is a nitride layer that acts as an etch stop during an etching step for removing the unwanted oxide trench fill material.
In some preferred embodiments, depositing the pad protective material includes depositing a first nitride layer over the substrate upper surface. Portions of this first nitride layer are selectively removed to expose portions of the substrate and to form sidewalls in the first nitride layer. Depositing the trench protective material preferably includes depositing a second nitride layer over the trench fill material and in contact with the sidewalls of the first nitride layer, such that the first nitride layer and portions of the second nitride layer covering the trench together form a continuous nitride layer.
In yet another preferred embodiment, the improved process includes depositing a pad protective material over the upper surface of the substrate, and selectively removing portions of the pad protective material to expose portions of the substrate and to form sidewalls in the pad protective material. A trench is formed in the exposed portions of the substrate, and a trench fill material is deposited in the trench and over the pad protective material. A trench protective material is deposited over the trench fill material and in contact with the sidewalls of the pad protective material, such that the pad protective material and portions of the trench protective material together form a continuous protective material layer. Portions of the trench protective material are selectively removed to expose portions of the trench fill material that overly the pad protective material, and the exposed portions of the trench fill material are selectively removed down to the level of the upper surface of the pad protective material. Finally, remaining portions of the pad protective material and trench protective material are removed, leaving the trench filled with trench fill material that is planarized at the upper surface of the substrate.
In another aspect, the invention provides a monolithic circuit manufactured according to a process as described above.